PPI vs. DPI: what’s the difference?

Though the terms DPI (dots per inch) and PPI (pixels per inch) both describe the resolution (or clarity) of an image, they’re not the same thing. PPI describes the number of square pixels that show up in an inch of digital screen (usually between 67-300). DPI, on the other hand, is a printing term referring to the number of physical dots of ink in a printed document.

Still not clear? There’s no shame in feeling lost on the DPI/PPI subject; the world seems to have conspired to make it as confusing as humanly imaginable. Among other reasons, this is largely a result of people (and some software manufacturers) using the two terms interchangeably.

Not to worry—by the end of this article you will have a thorough understanding of these terms and should feel comfortable enough to smugly correct any misuser of the terms. (And explain to them why they’re wrong! Because everyone loves that.)

Pixels Per Inch (PPI)
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We’ll start here because, if you’re a designer, your primary concern is going to be PPI. DPI, which we’ll discuss next, refers to a technical aspect of printing devices that doesn’t directly concern you — it’s the print shop’s domain.

People say “DPI” when they really mean “PPI” all the time — so much so that it’s become an established convention that you have to put up with (Apple, Microsoft and Adobe have all been guilty of this improper usage). The important thing to know is whether someone talking about DPI really means PPI. Read on and you will.

PPI’s digital basis

First off, what is a pixel? It seems rudimentary, but for many the confusion begins here. Pixel stands for “picture element”. It’s the smallest physical element of a digital display device that the eye can discern.

Zoom in close to the photo on your computer screen and you’ll see them: rows and rows of tiny little squares. These are also the smallest addressable unit of a digital image.

In fact, pixels are actually made up of “sub-pixels” — red, green and blue light elements that the human eye cannot see because additive color processing blends them into a single hue which appears on the pixel level. But this fact is not directly relevant to designers.

Confusion point: Regrettably, some manufacturers refer to these sub-pixels as “dots” because they are (roughly) analogous to the CMYK dots of a printer, which function in a similar way but by subtractive color processing (more on this later). These manufacturers then boast of the “DPI” of their screens. If you see this, ignore it! It is an annoying misuse of terms and probably an attempt to overcharge you.

Note that pixels are physical things of a fixed size (albeit not a standard one; different devices have pixels of different shapes and sizes, the smallest known being a microscopic 11¼ μm).

Hence, the number of pixels per inch (PPI) on your screen is a fixed quantity — not something you can adjust by typing in a new number somewhere. Most LCD monitors are in the neighborhood of 67 – 130ppi.

If you’re only going to look at an image on a screen, its PPI doesn’t matter because the PPI of your monitor is already fixed. So next time someone tells you to upload images to a website at 72ppi because that is “web resolution,” you can tell them that they have simply added a ridiculous extra step.

Unless they are concerned with visitors taking the images from the website and then printing them, the PPI doesn’t matter. A 72ppi image and a 3,000ppi image will appear exactly the same on screen.

What you need to understand: PPI and a printed target

So we’ve established this much: setting PPI only matters for printing — the transfer of a digital image onto a non-digital surface. “But wait…” you say, “we’re talking pixels per inch, but printing paper doesn’t have pixels!” Yep, it’s confusing. Read on.

In the printing process, all the physical pixels that composed the image on screen are translated into little squares of different hues on paper. Obviously these are not pixels in the sense of the light-emitting mechanical device but “pixels” in the more abstract sense of a square picture element (we’ll use quotes around this abstract usage from now on to help keep things clear).

What does this mean? “Pixels” on paper have no fixed size. If you increase the size of your image by 300%, the “pixels” on the paper will become three times as large, resulting in a bigger but more rough-looking image.

And how do you increase or decrease print-out size in this way? By adjusting the number in the PPI (or, depending on your software, DPI) field.

Suppose you have a 300 x 300 pixel image. If you set the PPI to 10, this is going to make the print out relatively large: at 10 pixels per inch, it will be 30 x 30 inches (300 divided by 10 is 30). If you set the PPI to 300, this is going to make the print out relatively small: at 300 pixels per inch, it will by 1 x 1 inches (300 divided by 300 is 1). Make sense?

Takeway: Think of the PPI input as a way to adjust the physical size – not the resolution – of the eventual print-out. Decreasing the PPI, thus increasing the size of the printout, may seem to produce a lower quality image because the pixels are larger and more visible.

But remember, this is only a relative gauge of quality; if you were to stand further away, the image would appear as clear as it did before. The absolute resolution of the image has not changed; there are still as many “pixels” relative to the picture as there were before. So the way to increase the resolution of an image is to produce an image with more pixels, not increase the PPI.

Note: simply re-sampling an image at a higher number of pixels (inputting a new number into the pixels field after the image is already made) is generally not a great way to go about increasing quality, because the computer will likely cram the image full of pixels in weird places.

Dots Per Inch (DPI)
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Printers do not reproduce an image by tiling pixel squares directly on top of one another. Rather, they reproduce an image by spitting out tiny dots consisting of a mix of four colors, Cyan, Magenta, Yellow and Key (black), which combine to create a range of hues by the subtractive color model. There is bound to be some space between these dots, and this is what DPI measures: their density.

For example, if you are printing a 150ppi image at 600dpi, each “pixel” will consist of 16 dots (600 dots/150 “pixels” = 4 rows of 4 dots per “pixel”).

This matters to the client because, as a rule of thumb, the higher the DPI, the better the image’s tonality and the smoother its color blending will be (it will also use more ink and take longer to print, so keep that in mind for personal home printing). 150dpi is generally considered the minimum standard for high quality photographic reproduction in books and magazines. Newspapers often use 85dpi and the effect is clear: individual dots are visible and some detail is lost. Billboards go as low as 45dpi, but you can’t tell because you’re typically viewing from very far away. Typical dot matrix printers are capable for 60 – 90dpi, inkjet printers 300 – 600dpi, and laser printers 600 – 1,800dpi.

Note: higher dpi does not necessarily equate to higher quality because there is no standard dot size or shape, meaning that one manufacturer’s dots might look as good at 1200dpi as another manufacturer’s dots do at 700dpi. Anyway, that’s not really your problem.

Takeaway: DPI is just a technical aspect of an individual printer, like the pixel resolution of your computer monitor. As a designer, you have no control over this. All you can do is recommend your client to a professional print shop and have the shop, which will know the specifications of its machines, take over from there.

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Any comments?

Displays are usually specified in MDPI, HDPI, XHDPI, XXHDPI – 160 is the logical density for MDPI devices, right? A Nexus 5 is specified with 445 ppi, which should or could be the same as 320 DPI / XDPI or 480 / XXHDPI? Is there any formula to convert ppi into dpi?
Image based on: https://www.youtube.com/watch?v=zhszwkcay2A

Thanks.
Modern Retinas have a density of 400ppi, my old notebook just 100ppi.
There is a rule out there that says to store images (for Retina) twice the size of the original Web image. Example: Image width = 1.000px.
1.) Is my understanding correct, that for iPhone6+ (400ppi display) you would need to store width = 4.000px?
2.) How will this large image (kB and px-size) be scaled to the right size on each device, automatically?

studio5

Feb 26 2013

Actually dpi/ppi DOES have an impact on the screen.

If you create a PDF with images of identical pixel extents but differing ppi metadata the result is a mess of different sized pages.

To see an image at its best it must be displayed at a pixel per pixel match… on a Macpro super retina 13″ for example this requires a ppi resolution of 227. Otherwise it will interpolate.

The latter is a real problem to do with the pixel doubling of the Retina solution which to prevent teeny tiny icons and so forth is treated for much of the time as a 1440×800 monitor with the applications graphics then pixel doubled.This really messes up video. If you want to zoom 1:1 on a video player on a Macpro retina…all I can say is good luck. This is proving a nightmare in my work. As far as I know there is no way to add ppi data to video and no player would recognise it if there were.

PDF stands for Portable Document Format. As it stands today, it is used
so that regardless of what platform you viewed it on it would still be
accessible (not pixel perfect but accessible). Originally it was
primarily used in desktop publishing, ie; print purposes. The fact that
it can be viewed on screen does not detract from its original core
purpose in printing. It is merely a nicety to user experience. So
although you are right about viewing pdfs on screen, you are still
incorrect about PDFs being an exception because you are ignoring its
core purpose for use in printing. In which case PDF is not solely for on
screen, which is what the article said. “If you’re only going to look at an image on a screen, its PPI doesn’t matter because the PPI of your monitor is already fixed.”

studio5

Feb 26 2013

Jim I repeat. Create a sequence of images of matching resolution, lets say 1929×1080 but set their DPI differently, some 72dpi some 300. Use adobe acrobat pro to assemble a PDF of these.

Now view the result in multiple operating systems. The result is a disaster. You have to match dpi across all images.
.

We are long past the era where PDF’s are used with the primary purpose being to print. PDF’s are open and used just about everywhere now as yet another reading medium. I don’t print off my 1300 page ebooks. Sometimes I am lucky and can get them as an epub or similar open-format originally intended for reading. However PDF’s are also commonly used because very rarely will you find a device that doesn’t have either Adobe reader or a free pdf reading alternative.

Drew Goldsberry

Feb 26 2013

Converting images to web optimized is really more to reduce the size of the image to 72 at the size you need to make it so the client can download the image quickly. This is not a worthless step, serving only what is needed not what is available can provide a better user experience. Nice post had some really great content- Web Developer

I suggest you try for yourself; make an image 72dpi and another 3000 dpi. Check the file size. It is the exact same. You are not providing a better user experience, you are indeed just adding an extra unnecessary step. The DPI/PPI setting only tells the printer how big you wan’t to print the image. If the image is shown on the web, it doesn’t matter the slightest.

Drew Goldsberry

Feb 26 2013

The assumption in my previous post was you were using the image inside a website. For instance I have space for a 400px wide image and i am starting with a 2000+ wide pixel image. I could allow the browser to resize the image for me or i could send a 400px wide 72 dpi image. Both would achieve the same effect the difference is the browser does not need to load the large image if it is only 400 px wide for instance. I understand where the confusion is i was not clear on my post.

Yes re-sampling an image without keeping the dimensions the same will result in the same size image.

Johan Dahl

Feb 26 2013

I’m not sure you understand. Serving a 2000+ wide pixel image would indeed be bad, since it would undoubtedly be of bigger size than a 400px image. BUT, the DPI doesn’t matter when it comes to web images. You don’t need to set your 400px wide image to 72 DPI (it can be anything). The DPI of that image is totally irrelevant when it comes to displaying it on the web.

Александр Компанец

Feb 26 2013

How can the DPI of an image be changed? Article says “DPI is just a technical aspect of an individual printer”.

At the same time author says “the PPI doesn’t matter” (not DPI but PPI). He says you may do your site using 3000 PPI images and it wil look the same. It will. But the size of that images will be too large.

Johan Dahl

Feb 26 2013

No it won’t. The size of the image will not change at all, which is easy to test for yourself! I suggest you do that.

Александр Компанец

Feb 26 2013

Got it! PPI affects only printed image. If we don’t change the resolution it will take the same disk space. Thank you. Really didn’t know that we may leave the resolution while changing PPI setting.

This bothered me as well, and it looks like the responses to this comment just muddied the waters. If you take a large image that is set at 800 pixels wide x 300 pixels hight at 300 ppi it will be a large file. In order to post the image online the convention is to reduce the ppi to 72 for bandwidth purposes.
I understand the argument but it over-complicates the situation. Most images have a set ppi when you open them in an image editing software like Photoshop, and this should be the frame of reference.

Mitch Gohman

Feb 26 2013

Actually a lot of people confuse this topic. I had trouble wrapping my head around it the first time I examined it. And now with retina displays (device pixels) – it ramps up the confusion.

It used to be that screens would have a resolution close to 72 pixels per inch. Physical inches. Nowadays, since people can set there display resolution to whatever they like, this is no longer valid (been over 30 years). 72 PPI is, however, a convention that people have been following for years as a result. But it actually doesn’t impact the file size. An 800×300 pixel image at 72 ppi is the same as a 800×300 pixel image at 300 ppi. Each image contains the same number of pixels. The only difference is when you go to print. PPI = printed pixels per inch.

The only semi-valid argument I have discovered in working with images for web, is that when you set the document to 72 PPI, and you work with Fonts – the font conversion of POINTS, is the same size as pixels (12 pts = 12 pixels). Because 72 points in print is equal to 1 inch. You could also just change the font settings to use pixels instead of points in Photoshop (or editor of choice). And not have to deal with the conversion at all.

At the end of the day – wether you set your document to 72 ppi or 300 ppi – it won’t make a difference. I always tell my students to use 72 ppi just to follow the convention and call it day. in other words, it doesn’t change the price of tea in China – and most people won’t get it any way. It also helps when working with others who do not understand this trivial fact. When you give them a file – they expect it to follow conventions. The agreed approach. When it doesn’t, you may spend more time trying to explain why it does not matter, than actually working on the website. 🙂

At the same time, when people get this misconception -it elevates their understanding of the files they are managing.

John Marovino

Feb 26 2013

Great article Alex . well written and badly needed. There’s so much confusion over a topic that shouldn’t be that complicated. Thanks.

You can add Amazon into the list of major companies who just can’t seem to grasp the difference between PPI and DPI. I’m having a discussion right now with Amazon reps who swear when you go into Photoshop and alter an image’s resolution that you’re changing its DPI – even though it says right there on the resolution screen that you’re changing the PPI. It’s not like it’s hidden. The words “pixels per inch” are right there next to the resolution entry box.

Hi there, I need some very basic advice – I am not a designer, but I came across this post because I am trying to make a photobook via Mixbook, and I looked up their help section to make sure my photos don’t print poorly. I have some photos I am trying to use that I have no digital copy of, so I was planning to scan them and edit them in photoshop, which I have never really used before.

Anyway, this is what the help section says: “For best print quality, photos should be at least 4MB at 300 dpi. The maximum file size at this time is 15MB.”

So, in photoshop I cannot see any way to edit DPI, which doesn’t surprise me after reading this post. But now what I am confused about is, how to I achieve these recommendations? Do they mean PPI? And what does the file size have to do with the print quality? Can I change that?

First, when you open a new document. Below “Height” and “Width” is an option for “Resolution”. That’s where you should edit your DPI (although Photoshop calls it “Pixels/Inch”).

You can also make changes later on by going to: Image > Image Size and edit the resolution there. Keep in mind that, because these are not vectors, you won’t be able to make the image larger or increase the resolution without some shrinkage or pixelation. The best plan of action is to start from the beginning with the size and resolution you want.

Roman

Feb 26 2013

so just to be clear, your own concept of ppi vs dpi is now backwards? In your article you claim that DPI is a physical aspect of a printer (which it is, it is the physical limitation a printer has to print an image) but now you say that by changing the resolution in photoshop (which photoshop calls Pixels/Inch, which is correct) you are actually changing the DPI. So do you yourself now not know which is which?

99designs

Feb 26 2013

Hi Roman,

We find that often when print shops request a certain “DPI”, what’s actually meant is PPI (via Photoshop). The standard for photo print shops is 300PPI resolution to print in good quality, which we assume is what was asked of Rachael. We’ve edited our response for clarification.

Tracy

Feb 26 2013

I am very confused. I am trying to create a large mural for a client. If I purchase the images through Shutterstock at their largest size say 330dpi and I need to print the mural and roughly 10′ x 10′ with a minimum of 72ppi it will be blurry so I am told. Is there a calculation I can use to determine how many dpi I need in order to achieve a clear image at 10′ x 10′ with 72-100ppi?

OK. I wanted to submit a photo to my school magazine. They requested that we submit the photo by e-mail as a 72dpi image, and if they want to print it in the magazine, they will ask us to submit to them the 300dpi image. So do they really mean ppi and not dpi? Is dpi something the school magazine would have to manipulate with the printer? Also, is Photoshop the only way to change dpi/ppi? Does a DSLR camera allow you to change that in its settings somewhere?

Check out ImageMagick (if you are not shy around the command prompt or ‘terminal’ depending on your OS).

On windows (and possibly linux if you have Mono) you can also look into Paint.NET. It is a free tool that lets you alter information about images (such as re-sampling the image, altering the DPI/PPI, etc).

Certain tools that attempt to show you what something would look like printed (because monitors are very rarely, if ever, going to print an image the same size your monitor displays it at) will use the DPI/PPI to actually ‘print’ the image (and it’s surrounding text/objects) to something such as a PDF or other transferrable media; to do so, it has to mimic a printer and will take the DPI of your image into effect.

This also can be seen when using web-tools to ‘create’ your own business cards, ‘design’ your own t-shirts, etc.

Also, some tools, will take into consideration the DPI/PPI in attempts to display the image at (or as close to as possible) to what you would see if you printed it, in effect emulating the printer.

arbee

Feb 26 2013

This is a very good article, but I do have a question. You say…. “If you’re only going to look at an image on a screen, its PPI doesn’t matter because the PPI of your monitor is already fixed. So next time someone tells you to upload images to a website at 72ppi because that is “web resolution,” you can tell them that they have simply added a ridiculous extra step.”
I understand that it won’t make any difference to viewing the image, however does it make a difference to the file size?

of course they’re will be different in size, because more pixels = more size.

James

Feb 26 2013

Except that, with JPEGs, compression varies the number of pixels (file size), but not the size of the image on a display! Trying to explain this stuff to normal human beings is impossible!

BourbonAndCoke

Feb 26 2013

Pixels exist on your monitor, not in a JPEG file. Compression has nothing to do with this subject. A 1000px X 1000px image is still 1000px X 1000px with or without compression.

James

Feb 26 2013

Correct in principle, but a heavily compressed JPEG will be unrecognizable on a screen or a printed image, as the compression removes some of the details. Once compressed, those bits of data are gone, and the file is smaller, although it would still appear the same size displayed on a screen.

BourbonAndCoke

Feb 26 2013

Well, correct in fact and in practice as well! Your comments are also true, but JPG compression is a distinct issue from the topic of this article and comments. The way Adobe labels and presents the DPI/PPI/resolution options is what I think adds greatly to the confusion about the issue and has for years. It took some real headaches when I was starting out to figure out how the different settings actually worked.

The funny thing is, right now, for the past week, I’ve been editing images for use on a client’s e-commerce site, and changing the ppi to 1600 (!) on each image, because that’s what the customer insisted on. I told him once that this is an unnecessary step that doesn’t affect anything. He looked at me with a blank stare, so I said “o.k.! 1600 it is!”

BourbonAndCoke

Feb 26 2013

Fail. There will not be a difference in size, which is the whole point of this article. Changing a Photoshop setting from 72 ppi to 15000 ppi will make zero change to the image. The two images are identical, the same, not different, not larger, not bigger file size, not more pixels, not “of course.”

Karl

Feb 26 2013

Of course it does. Open any image in Photoshop and change it from 72 ppi to 15000 or whatever. You get a larger image, more pixels, bigger file size, all of that.

BourbonAndCoke

Feb 26 2013

You get a larger image, more pixels, bigger file size, etc. if you have the option “resample” (resize) checked, in which case Photoshop calculates what the width and height need to be in order to *print* at the so-called “resolution” you’ve set. That resizes the image accordingly. But that scenario does not apply here because you’ve added a second consideration by explicitly telling PS to resize the image. What I’m saying is that changing the PPI *without* resizing the image will result in an identical image. Same file size, same dimensions, “all of that.” 🙂

Karl

Feb 26 2013

You’re right. And Galih was also right, considering that Photoshop resamples the image by default. So you must tell it “explicitly” that you don’t want to resize the image. What a disappointment. I was thinking I could turn my lousy 5 MP phone pictures into glorious high-res 30 MP images, so I don’t have to buy a fancy camera.

BourbonAndCoke

Feb 26 2013

Before you give up hope, consider that I started an all-digital studio/event photography business in 1999, paying $1000 for *1MP* Kodak camera, and I made prints as large as 20″ x 30″ for store displays (after much PS work) when even 8″ x 10″ was a stretch. I wouldn’t be as concerned with the image size as I would the overall quality of the camera on your phone, since they’re often pretty lousy. But test it out! Take your best shot, interpolate it as you need to, and get a test print at Walmart. When I started that business there was ONE place in town to get photographic quality digital prints, and an 8″ x 10″ would cost you $20. You don’t have it so bad!

Shraddha Rathi

Feb 26 2013

Thank you ever so much for the information.
Resolved my problem …well at least I realized that I really don’t have a problem when the publisher asked for different resolution files for web post!
Thank you again

Thanks very much! But I am a little confused about it. I interpreted your information as PPI is an affair of the computer monitor manufactures’, whereas DPI is an affair of printer manufactures’, so it seems there is nothing concerned with us users. It is right? So could if we change the PPI/DPI of some picture file? And I am concerned with one thing, how to determine the pixels of a certain size of letters?

So should I export a digital image in a similar amount of PPI as the printer will use DPI?… Or does that not matter as long as it’s not exported with a too low DPI?
For example; If I know a print will be made at 300dpi, and I have an image exported at 480ppi, is that a problem?

A chart can be a bit tough since there are a variety of types of screens available (smartphone, tablet, laptop, TV, monitor, etc) all at different sizes. Figuring out the PPI of a screen is basically a math equation.

First to clear up the terms:

For screens, display resolution refers to the number of pixels on the screen itself, how many pixels high vs how many pixels wide. Display size is the physical size of the screen in inches. So the relationship between PPI and resolution means that PPI (pixel density) depends on the total number of pixels the screen has (display resolution) within the physical measurements of the screen (display size).

The formula is a bit complicated, but there are handy calculators available like these: http://kingscalculator.com/en/other-calculators/pixel-density-calculator

janet

Feb 26 2013

Coming from the print world, dots in printing are a fixed size, and only variable in how many appear per inch — determining the degree of image detail.

I suppose that’s a way of looking at it, like cells that make up the body.

Bennett Haselton

Feb 26 2013

I learned some useful information here but I think this is still potentially confusing unless you clarify something right at the top: There are *two different* definitions of “PPI” being used in the article.

When you refer to monitor PPI, you’re talking about the physical pixels per inch of monitor space (something that can’t be changed).

When you refer to the PPI of an image file, you’re talking about a file attribute that is a directive to the printer (not the monitor) telling it how many pixels per inch to use when printing the file (which of course can be changed).

Your first couple of paragraphs are referring to the PPI of the monitor, but then the first time you say “its PPI doesn’t matter,” that’s the first usage of “PPI” that refers to the PPI attribute of an image file.

The complete explanation as I understand it, is:
– There is the PPI attribute of a monitor as described above
– There is the PPI attribute of an image as described above
– By default, when an image is displayed on a monitor, there is a 1-to-1 mapping between the logical pixels of the image and the physical pixels of the screen, because this is the optimal display. But they are not *definitionally* the same, and you can change this by changing the resolution of your display.
– A display screen is supposed to ignore the PPI attribute of an image file completely. For two images whose pixels are identical, but one is set to 300 PPI and the other is set to 150 DPI, they should be displayed at the same size on the screen by default. But when you print them, one will be twice as large as the other.

Converting pictures to net optimized is de facto a lot of to scale back the dimensions of the image to seventy two at the dimensions you wish to create it that the shopper will transfer the image quickly. this can be not a manky step, serving solely what’s required not what’s accessible will give a more robust user expertise. Nice post had some extremely nice content- net Developer

With one very painful exception to all this, Microsoft. PowerPoint and Outlook both read the DPI of the file and use that when insterting graphics. I’ll drop you one life line, use 96dpi when making graphics for emails in Outlook.